Translesion synthesis (TLS) in Saccharomyces cerevisiae requires at least Rev1p and polymerase zeta (Pol zeta), a complex of the Rev3 polymerase and its accessory factor Rev7p. Although their precise role(s) are poorly characterized, in vitro studies suggest that each protein contributes to TLS in a manner dependent on the particular lesion and surrounding DNA sequence. In the present study, strand segregation analysis is used to attempt to identify the role(s) of the Rev1 and Rev7 proteins during TLS. This assay uses double-stranded plasmids containing a genetic marker opposite to a replication blocking lesion (N-2-acetylaminofluorene; AAF) to measure TLS quantitatively and qualitatively in vivo. The AAF adduct is localized within a repetitive sequence in a manner that allows the formation of misaligned primer-template replication intermediates. Elongation from a misaligned intermediate fixes a frameshift mutation (slipped TLS), while extension of the correctly aligned lesion terminus yields error-free (non-slipped) TLS. The results indicate that there is a strong requirement for Rev7p during Pol zeta-mediated TLS measured in vivo. Furthermore, Rev1p is needed only for non-slipped TLS; slipped TLS remains efficient in its absence, revealing a previously uncharacterized Rev1p activity similar to Escherichia coli UmuDC function. Specifically, this activity is required for elongation from a correctly aligned lesion terminus.